QR code keychains: sizes, the ring hole and durability

By Tiehl Roberto Matías · 6 min read · Updated 25 October 2026

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The keychain is the most requested piece and the easiest to get wrong: it is the smallest, so it is where module size bites hardest, and on top of that you have to put a hole in it without eating into the code's quiet zone. These are the measurements that work.

The size: 40 mm is the balance point

A keychain has to be small enough for a pocket and big enough for the code to read. The balance sits between 40 and 50 mm a side, and the reason is arithmetic.

For different content or a different nozzle, the size calculator (in Spanish) gives you the exact side before you spend any filament.

With a 0.4 mm nozzle the module should not drop below 0.8 mm. A version 3 code (29 × 29 modules) plus its eight-module quiet zone needs 37 modules of width, which at 0.8 mm is 29.6 mm. Add the tab for the ring and a bit of edge and you are at a long 40 mm.

Keychain side	Comfortable maximum version	Characters in the link (level M)
35 mm	2 (25 × 25)	up to 26
40 mm	3 (29 × 29)	up to 42
45 mm	4 (33 × 33)	up to 62
50 mm	5 (37 × 37)	up to 84
60 mm	7 (45 × 45)	up to 122

Below 35 mm the keychain only works for very short links. If yours does not fit, the answer is not to squeeze the code: it is to shorten the address.

The small version of this piece, with the same arithmetic but with the size decided by the animal wearing it, is the collar tag (in Spanish).

The ring hole

This is where most home-made keychains go wrong. The hole gets put inside the square so as not to make the piece longer, and doing that eats part of the quiet zone or even modules of the code itself.

Measurements that work:

• Hole diameter: 4 mm. Any standard keyring fits, which is usually 25 mm outside diameter and 1.5 mm wire. At 3 mm it is a struggle to get in; at 5 mm you lose material around it.
• Material between the hole and the edge: 2.5 mm minimum. That is the part that breaks. Under 2 mm, the keychain snaps there inside a month on a bunch of keys.
• The tab sticks out 7-8 mm from the square of the code. Enough for the hole and its wall, without the piece looking out of proportion.

The thickness

A keychain spends its life banging against metal keys inside a pocket. It needs more body than you would think.

• Base: 2.5 to 3 mm. At 2 mm the keychain flexes and ends up cracking around the hole. At 3 mm it does not break.
• Relief: 0.6 mm. Less than standard, and deliberately so: on a piece this small the wall of the relief hides the light modules as soon as the phone tilts, as the relief height guide (in Spanish) explains. It is still enough to keep the code from rubbing against the keys.
• Total thickness: 3.5 to 4 mm.

The keychain is the exception. On almost every other piece it pays to bring the base down, because it takes 90 % of the weight and the extra stiffness is not needed; here it is, because the ring hole is a weak point. The rest of the cases are in the base thickness guide (in Spanish).

Slicer settings

For a small piece that is going to take a beating, it pays to step away from the fast profile:

Setting	Value	Why
Perimeters	4	The strength of a flat piece is almost all in the perimeters, not the infill.
Infill	40-50 %	Below 30 % the piece gives when you bend it.
Infill pattern	Gyroid or rectilinear	They spread the load better than the fast line pattern.
Top layers	5	The code is on the top face: with too few solid layers, the infill shows through.
Layer height	0.2 mm	Divides a 3 mm base exactly and gives a clean colour change.

Orientation on the plate

Flat, with the code facing up. Always. Three reasons:

• It is the orientation in which the layer colour change works.
• The layers end up perpendicular to the bending load, which is how they hold best.
• No supports are needed anywhere.

Printing it on edge to «save time» produces a piece that snaps along the first layer that gets pulled, and forces supports under the hole as well.

How many fit per print

Here is the real advantage of this piece. A 40 mm keychain plus its tab takes about 48 × 40 mm. On a 256 × 256 mm bed, leaving 5 mm between pieces:

• Five columns by four rows: 20 keychains on one plate.
• With a 3 mm base and 1 mm relief, that is 6.1 grams a piece, 122 for the whole plate.
• And a single filament change covers all twenty, because they all have the base at the same height. That is why the QR keychain is such a rewarding thing to make for fairs and events.

How many keychains fit on each bed size, and what the whole plate weighs and takes in each case, is in the batch printing guide (in Spanish).

If you are going to hand them out

Two pieces of advice learnt the hard way:

• Make the link yours, and make it redirect. A keychain lasts years on a bunch of keys. If it points straight at a URL on one particular platform and tomorrow you move platform, you have handed out a hundred dead pieces. If it points at an address on your own domain that redirects, you change the redirect and they all keep working.
• Print one and run it past three phones before committing to the run. One of yours, one of another brand and ideally one old one. Ten minutes that save throwing away a whole plate. The full order of checks is in the testing guide (in Spanish).